The invention relates to a brushless motor for rotating a disk or the like.
Recently, in a brushless motor for rotating a disk or the like, a waveform shaping circuit is used for detecting a rotation of the motor in order to measure a rotational speed of the motor. As disclosed in, for example, the unexamined published Japanese patent application (TOKKAI) SHO 63-256013, the waveform shaping circuit is configured so as to produce a pulse signal by shaping an alternating signal of a frequency proportional to the rotational speed of the motor.
FIG. 27 shows configurations of a detector and the waveform shaping circuit which are used in detection of the rotation in the prior art. In an optical disk 2002 attached to a rotor shaft 2001, slits are formed at regular angular intervals. A light emitting diode 2010 and a phototransistor 2011 are attached to a support member 2003 fixed to the stator, so as to face each other across the slits of the optical disk 2002. A detection signal from the phototransistor 2011 is amplified by a linear amplifier circuit 2020, and an alternating signal 2101 is output. Accordingly, in response to the rotation of the optical disk 2002, the alternating signal 2101 of a frequency which is proportional to the rotational speed of the rotor is generated in the phototransistor 2011 and the amplifier circuit 2020.
A first comparator block 2110 having no hysteresis receives the alternating signal 2101, and produces a shaped signal 2141 which is obtained by comparing and shaping the alternating signal 2101. The first comparator block 2110 comprises a comparator 2111, an inverting circuit 2112, and a DC voltage source 2113. A second comparator block 2120 having a predetermined hysteresis width corresponding to resistors 2121 and 2122 produces another shaped signal 2142 which is obtained by comparing and shaping the alternating signal 2101 by using the predetermined hysteresis width. The second comparator block 2120 comprises the resistors 2121 and 2122, a comparator 2123, and an inverting circuit 2124. The shaped signals 2141 and 2142 are supplied to a pulse generating block 2130. The pulse generating block 2130 comprises an inverting circuit 2131, an AND circuit 2132, and OR circuits 2133 and 2134. The pulse generating block 2130 outputs a pulse signal 2150, the level of which is changed during a time period from a leading edge of the shaped signal 2141 to that of the shaped signal 2142. As a result, the rotational speed of the motor can be measured on the basis of the pulse signal 2150.
However, in the conventional brushless motor, many detection parts and detection devices are required to attach to the rotor and the stator in order to generate the alternating signals. Consequently, the number of mechanical parts is large and the production of the motor is complicated. In particular, the optical disk 2002 having slits must be attached to the rotor, and the light emitting diode 2010 and the phototransistor 2011 must be attached to the stator. As a result, the conventional brushless motor has problems that the production cost is high and that the space for such parts is large.
In the conventional brushless motor, moreover, the pulse signal 2150 can be used for measurement of the rotational speed but cannot be used for detection of a rotational direction. In many applications for rotating the disk or the like, it is required also to detect the rotational direction. In the above-mentioned conventional configuration, some detection parts for detecting the rotational direction must be additionally disposed.
As the two comparator blocks 2110 and 2120 are used for shaping the same alternating signal 2101, the circuitry of the conventional brushless motor has a complex configuration. In order to simplify the circuitry, it may be contemplated that only the first comparator block 2110 having no hysteresis is used, and the second comparator block 2120 and the pulse generating block 2130 are omitted. However, since the first comparator block 2110 has no hysteresis, many noise pulses are generated at timings when the edges of the shaped signal 2141 are generated, by high-frequency noises contained in the alternating signal 2101. The pulse signal from the first comparator block 2110 cannot be used as the signal for measuring the rotational speed.
On the contrary, in order to simplify the circuitry, it may be contemplated that only the second comparator block 2120 having hysteresis is used, and the first comparator block 2110 and the pulse generating block 2130 are omitted. However, since the second comparator block 2120 has hysteresis, timings of the edges of the shaped signal 2142 are shifted from the respective zero-crossing points of the input alternating signal 2101 by a value corresponding to the hysteresis width. Both the pulse width and the pulse period of the shaped signal 2142 are varied by an amplitude modulation component contained in the alternating signal 2101. Therefore, the pulse signal of the second comparator block 2120 cannot be sufficiently as the signal for measuring the rotational speed.
Although the two comparator blocks are used in the conventional brushless motor, the output signal has noise pulses when the alternating signal 2101 contains noises larger than the hysteresis width of the second comparator block 2120. In mass production, there are large variations in the amplitude of the alternating signal 2101. Therefore, the hysteresis width must be set to a value sufficiently smaller than the estimated minimum amplitude in mass-produced motors, and the margin for noises is reduced remarkably.